Eileen M. Cullen

Performance and prospects of Rag genes for management of soybean aphid

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an invasive insect pest of soybean [Glycine max (L.) Merr. (Fabaceae)] in North America, and it has led to extensive insecticide use in northern soybean‐growing regions there. Host plant resistance is one potential alternative strategy for managing soybean aphid. Several Rag genes that show antibiosis and antixenosis to soybean aphid have been recently identified in soybean, and field‐testing and commercial release of resistant soybean lines have followed. In this article, we review results of field tests with soybean lines containing Rag genes in North America, then present results from a coordinated regional test across several field sites in the north‐central USA, and finally discuss prospects for use of Rag genes to manage soybean aphids. Field tests conducted independently at multiple sites showed that soybean aphid populations peaked in late summer on lines with Rag1 or Rag2 and reached economically injurious levels on susceptible lines, whereas lines with a pyramid of Rag1 + Rag2 held soybean aphid populations below economic levels. In the regional test, aphid populations were generally suppressed by lines containing one of the Rag genes. Aphids reached putative economic levels on Rag1 lines for some site years, but yield loss was moderated, indicating that Rag1 may confer tolerance to soybean aphid in addition to antibiosis and antixenosis. Moreover, no yield penalty has been found for lines with Rag1, Rag2, or pyramids. Results suggest that use of aphid‐resistant soybean lines with Rag genes may be viable for managing soybean aphids. However, virulent biotypes of soybean aphid were identified before release of aphid‐resistant soybean, and thus a strategy for optimal deployment of aphid‐resistant soybean is needed to ensure sustainability of this technology.

Insect growth regulator effects of azadirachtin and neem oil on survivorship, development and fecundity of Aphis glycines (Homoptera: Aphididae) and its predator, Harmonia axyridis (Coleoptera: Coccinellidae)

BACKGROUND Aphis glycines Matsumura, an invasive insect pest in North American soybeans, is fed upon by a key biological control agent, Harmonia axyridis Pallas. Although biological control is preferentially relied upon to suppress insect pests in organic agriculture, approved insecticides, such as neem, are periodically utilized to reduce damaging pest populations. The authors evaluated direct spray treatments of two neem formulations, azadirachtin and neem seed oil, under controlled conditions for effects on survivorship, development time and fecundity in A. glycines and H. axyridis. RESULTS Both azadirachtin and neem seed oil significantly increased aphid nymphal mortality (80 and 77% respectively) while significantly increasing development time of those surviving to adulthood. First-instar H. axyridis survival to adulthood was also significantly reduced by both neem formulations, while only azadirachtin reduced third-instar survivorship. Azadirachtin increased H. axyridis development time to adult when applied to both instars, while neem oil only increased time to adult when applied to first instar. Neither neem formulation affected the fecundity of either insect. CONCLUSIONS Results are discussed within the context of future laboratory and field studies aimed at clarifying if neem-derived insecticides can be effectively integrated with biological control for soybean aphid management in organic soybeans.

One gene versus two: A regional study on the efficacy of single gene versus pyramided resistance for soybean aphid management

ABSTRACT The soybean aphid (Aphis glycines Matsumura) is a threat to soybean production in the Midwestern United States. Varieties containing the Rag1 soybean aphid resistance gene have been released with limited success in reducing aphid populations. Furthermore, virulent biotypes occur within North America and challenge the durability of single-gene resistance. Pyramiding resistance genes has the potential to improve aphid population suppression and increase resistance gene durability. Our goal was to determine if a pyramid could provide improved aphid population suppression across a wide range of environments.Weconducted a small-plot field experiment across seven states and three years. We compared soybean near-isolines for the Rag1 or Rag2 gene, and a pyramid line containing both genes for their ability to decrease aphid pressure and protect yield compared with a susceptible line. These lines were evaluated both with and without a neonicitinoid seed treatment. All aphid-resistant lines significantly decreased aphid pressure at all locations but one. The pyramid line experienced lower aphid pressure than both single-gene lines at eight of 23 location-years. Soybean aphids significantly reduced soybean yield for the susceptible line by 14% and for both single-gene lines by 5%; however, no significant yield decrease was observed for the pyramid line. The neonicitinoid seed treatment reduced plant exposure to aphids across all soybean lines, but did not provide significant yield protection for any of the lines. These results demonstrate that pyramiding resistance genes can provide sufficient and consistent yield protection from soybean aphid in North America.

Efficacy and nontarget effects of reduced-risk insecticides on Aphis glycines (Hemiptera: Aphididae) and its biological control agent Harmonia axyridis (Coleoptera: Coccinellidae).

Abstract The efficacy of three reduced-risk insecticides (pyrethrins, insecticidal soap, and narrow-range mineral oil) was determined for nymphs and adults of the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), an exotic pest of North American soybean, Glycine max (L.) Merr. These insecticides also were evaluated for nontarget effects on one of the aphid’s key biological control agents, multicolored Asian lady beetle, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), including first and third instars, pupae, and adults. A Potter Spray Tower was used to conduct direct spray laboratory bioassays. Results indicated that although pyrethrins and narrow-range mineral oil caused 100% mortality to A. glycines nymphs and adults at 72 h posttreatment, insecticidal soap caused equivalent mortality to only the nymphs during the same time period. However, A. glycines adult mortality due to the insecticidal soap (83.3%) was significantly greater than the control. Pyrethrins were highly toxic to first instars of H. axyridis (98% mortality), but they had no effect on third instars, pupae, or adults. Mineral oil and insecticidal soap were moderately lethal to first (48.9 and 40% mortality, respectively) and third (31.9 and 38.8% mortality, respectively) instars of H. axyridis, but they had no effect on pupae and adults. Our results suggest that pyrethrins, insecticidal soap, and narrow-range mineral oil may prove useful for soybean aphid management in organic soybean due to efficacy against the aphid with differential nontarget effects on select stages of H. axyridis. Additional studies will be necessary to elucidate the efficacy of these insecticides under field conditions.

Resistance to Bt Corn by Western Corn Rootworm (Coleoptera: Chrysomelidae) in the U.S. Corn Belt

Transgenic Bt corn hybrids that produce insecticidal proteins from the bacterium Bacillus thuringiensis Berliner have become the standard insect management tactic across the U.S. Corn Belt. Widespread planting of Bt corn places intense selection pressure on target insects to develop resistance, and evolution of resistance threatens to erode benefits associated with Bt corn, such as reduced reliance on conventional insecticides. Recognizing the threat of resistance, the U.S. Environmental Protection Agency requires seed companies to include an insect resistance management (IRM) plan when registering a Bt trait. The goal of IRM plans is to delay Bt resistance in populations of target insects. One element of IRM is the presence of a non-Bt refuge to maintain Bt-susceptible individuals within a population, and growers are required to implement IRM on-farm by planting a refuge. Field-evolved resistance has not been detected for the European corn borer, Ostrinia nubilalis (Hubner), even though this species has been exposed to Bt proteins common in U.S. corn hybrids since 1996. The IRM situation is unfolding differently for Bt corn targeting the western corn rootworm, Diabrotica virgifera virgifera LeConte. In this article, we examine the scientific evidence for D. v. virgifera resistance to Bt rootworm traits and the cropping system practices that have contributed to the first reports of field-evolved resistance to a Bt toxin by D. v. virgifera . We explain why this issue has developed, and emphasize the necessity of an integrated pest management approach to address the issue.

Spatial Distribution of Aphis glycines (Hemiptera: Aphididae): A Summary of the Suction Trap Network

ABSTRACT The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an economically important pest of soybean, Glycine max (L.) Merrill, in the United States. Phenological information of A. glycines is limited; specifically, little is known about factors guiding migrating aphids and potential impacts of long distance flights on local population dynamics. Increasing our understanding of A. glycines population dynamics may improve predictions of A. glycines outbreaks and improve management efforts. In 2005 a suction trap network was established in seven Midwest states to monitor the occurrence of alates. By 2006, this network expanded to 10 states and consisted of 42 traps. The goal of the STN was to monitor movement of A. glycines from their overwintering host Rhamnus spp. to soybean in spring, movement among soybean fields during summer, and emigration from soybean to Rhamnus in fall. The objective of this study was to infer movement patterns of A. glycines on a regional scale based on trap captures, and determine the suitability of certain statistical methods for future analyses. Overall, alates were not commonly collected in suction traps until June. The most alates were collected during a 3-wk period in the summer (late July to mid-August), followed by the fall, with a peak capture period during the last 2 wk of September. Alate captures were positively correlated with latitude, a pattern consistent with the distribution of Rhamnus in the United States, suggesting that more southern regions are infested by immigrants from the north.

Effect of organic, conventional and mixed cultivation practices on soil microbial community structure and nematode abundance in a cultivated onion crop

BACKGROUND Responses of the soil microbial and nematode community to organic and conventional agricultural practices were studied using the Teagasc Kinsealy Systems Comparison trial as the experimental system. The trial is a long-term field experiment which divides conventional and organic agriculture into component pest-control and soil treatment practices. We hypothesised that management practices would affect soil ecology and used community level physiological profiles, microbial and nematode counts, and denaturing gradient gel electrophoresis (DGGE) to characterise soil microbial communities in plots used for onion (Allium cepa L.) cultivation. RESULTS Microbial activity and culturable bacterial counts were significantly higher under fully organic management. Culturable fungi, actinomycete and nematode counts showed a consistent trend towards higher numbers under fully organic management but these data were not statistically significant. No differences were found in the fungal/bacterial ratio. DGGE banding patterns and sequencing of excised bands showed clear differences between treatments. Putative onion fungal pathogens were predominantly sequenced under conventional soil treatment practices whilst putative soil suppressive bacterial species were predominantly sequenced from the organic pest-control treatment plots. CONCLUSION Organic management increased microbial activity and diversity. Sequence data was indicative of differences in functional groups and warrants further investigation.

Potato Leafhopper (Hemiptera: Cicadellidae) Ecology and Integrated Pest Management Focused on Alfalfa

This article summarizes the knowledge to date on biology of the potato leafhopper, Empoasca fabae (Harris), including its distribution, development, migration, agricultural host plants, and mechanics of injury to host plants. Damage to alfalfa, potatoes, soybeans, and snap beans, and treatment guidelines, are summarized. Particular attention is given to integrated pest management options in alfalfa, the host plant most frequently incurring economically damaging populations of potato leafhopper. Alfalfa scouting and economic thresholds are discussed along with cultural controls and host plant resistance.

Conventional and Organic Soil Fertility Management Practices Affect Corn Plant Nutrition and Ostrinia nubilalis (Lepidoptera: Crambidae) Larval Performance

ABSTRACT Few studies compare how different soil fertilization practices affect plant mineral content and insect performance in organic systems. This study examined: 1) The European corn borer, Ostrinia nubilalis (Hübner), larval response on corn (Zea mays L.) grown in field soils with different soil management histories; and 2) resilience of these plants to O. nubilalis herbivory. Treatments included: 1) standard organic— organically managed soil fertilized with dairy manure and 2 yr of alfalfa (Medicago sativa L.) in the rotation; 2) basic cation saturation ratio—organically managed soil fertilized with dairy manure and alfalfa nitrogen credits, plus addition of gypsum (CaSO4·2H2O) according to the soil balance hypothesis; and 3) conventional—conventionally managed soil fertilized with synthetic fertilizers. Corn plants were reared to maturity in a greenhouse, and then infested with 0–40 O. nubilalis larvae for 17 d. O. nubilalis exhibited negative competitive response to increasing larval densities. Mean development time was significantly faster for larvae consuming basic cation saturation ratio plants than those on standard organic plants, with intermediate development time on conventional plants. Neither total yield (number of kernels) nor proportion kernels damaged differed among soil fertility treatments. Soil nutrients differed significantly in S and in Ca:Mg and Ca:K ratios, but principal components analysis of plant tissue samples taken before O. nubilalis infestation showed that S, Fe, and Cu contributed most to differences in plant nutrient profiles among soil fertility treatments. Results demonstrate that different fertilization regimens can significantly affect insect performance within the context of organic systems, but the effects in this study were relatively minor compared with effects of intraspecific competition.

European corn borer oviposition response to soil fertilization practices and arbuscular mycorrhizal colonization of corn

Soil fertility and resulting crop plant nutrition contribute to optimal crop yields in both conventional and organic farming systems. Additionally, soil management practices can affect the colonization and efficacy of arbuscular mycorrhizae (AM), which in turn may improve crop resilience to drought and soil nutrient deficiencies. Soil mineral fertilization and AM colonization have been shown to affect herbivorous insect oviposition response and performance. However, the below-ground interaction of fertilization practices and AM colonization on plant nutrition and insect oviposition response has been largely unexplored. To test this, we obtained soils from agricultural fields managed under 3 different soil fertilization practices for 5 continuous years: Synthetic fertilizers only with a 2-year corn-soybean rotation (conventional farming, or CONV), dairy manure with a 4-year alfalfa/oat-alfalfa-corn-soybean rotation (standard organic farming, or STDO), and dairy manure + 4-year alfalfa/oat-alfalfa-corn-soyb...